I've included a picture below. I just noticed this today, and it's weird looking and has a waxy feel. Not sure if you can see it in the picture, but there's also these weird reddish caviar looking things.

It's isolated in this one spot on the wood, and there's no indication of leakage or water intrusion.

theres definitely something going on moisture related. its either a leaking pipe dripping on it, water coming from below or some sort of animal using that spot as a bathroom. keep a close eye on it for a couple weeks to see if you can pinpoint it.. the wood itself wont do that